Photocatalytic reactions using the titanium oxide catalyst have been the focus of our research as an environmentally-friendly and safe means of converting light energy into useful chemical energy at ordinary temperatures without generating any pollutants. Investigations into the application of such photocatalysts for reactions that can decontaminate and detoxify the environment have been one of the most significant objectives. In particular, investigations into particle size reduction to form ultrafine particles as well as the addition of metals such as Pt, Ag, or Ni into the titanium oxide catalyst have enhanced the efficiency of the reactions. Furthermore, parallel investigations on the adsorption of a dye onto titanium oxide catalysts have been carried out to explore the possibility of the use of visible light (about 400 nm-800 nm).
However, until now, photocatalytic reactions in prior art photocatalysts could proceed only when the reaction systems were irradiated with ultraviolet-light in wavelength regions shorter than about 380 nm, but could not proceed constantly in the visible region having long wavelengths. Examples of photocatalytic reactions using prior art titanium oxide photocatalysts include a decomposition reaction of nitrogen oxides and an isomerization reaction of butenes. However, these photocatalytic reactions proceed efficiently only when the reaction systems are irradiated with ultraviolet light.
Titanium oxide catalysts do not allow for the use of visible light and make use of only about 5% of solar beams that reach the earth, necessitating the use of an ultraviolet light source such as a mercury lamp.